Humans once spent their nights in relative darkness. No longer. When the sun sets, TVs, computers, mobile devices, and artificial lighting burn on. The May issue of the Harvard Health Letter reports that this aspect of modern life may be great for efficiency, but not for health. At night, light throws the body’s biological clock—the circadian rhythm—out of whack. Sleep suffers. The combination of poor sleep and exposure to artificial light exposure may contribute to a number of health problems.

Studies have linked working the night shift and getting exposed to light at night to several types of cancer (including breast and prostate cancer), diabetes, heart disease, and obesity. It’s not exactly clear why nighttime light exposure seems to be problematic. It could be because exposure to light at night curbs the secretion of melatonin, a hormone that influences circadian rhythms.

But all light is not created equal, says the Health Letter. Blue wavelengths—which are beneficial during daylight hours because they boost attention, reaction times, and mood—seem to be the most disruptive at night. While light of any kind can suppress the secretion of melatonin, blue light does so more powerfully. In an experiment, researchers exposed people to 6.5 hours of light—either blue or green. The blue light suppressed melatonin for about twice as long as the green light and shifted circadian rhythms by twice as much.

While fluorescent lightbulbs and LED lights are much more energy-efficient than incandescent lights, they also tend to produce more blue light. That means the proliferation of electronic devices with screens, as well as energy-efficient lighting, is increasing exposure to blue wavelengths, especially after sundown.

What can you do? The editors of the Harvard Health Letter make the following recommendations:

Use dim red lights for nightlights. Red light has the least power to shift circadian rhythm and suppress melatonin.
Avoid looking at brightly lit screens beginning two to three hours before bed.
If you work a night shift or use a lot of electronic devices at night, consider wearing blue-blocking glasses.
Expose yourself to lots of bright light during the day, which will boost your ability to sleep at night, as well as your mood and alertness during daylight.